FIG. 1 is a view illustrating a network architecture of an Evolved Universal Terrestrial Radio Access Network (E-UTRAN), which is a mobile communication system to which the related art and the present invention are applied. The E-UTRAN system has evolved from the existing UTRAN system, and a basic standardization work thereof is currently going on in 3GPP. The E-UMTS system may be also referred to as a Long Term Evolution (LTE) system.
The E-UTRAN includes a plurality of e-NBs (e-NodeB; hereinafter, referred to as “base station”), and the plurality of eNBs are connected to one another through an X2 interface. The eNB is connected to user equipment (hereinafter, referred to as “UE”) via a wireless interface, and connected to an Evolved Packet Core (EPC) through an S1 interface.
The EPC may include a Mobility Management Entity (MME), a Serving-Gateway (S-GW), and a Packet Data Network-Gateway (PDN-GW). The MME has information on the connection of UE or the capability of UE, and such information is primarily used for the mobility management of the UE. The S-GW is a gateway having E-UTRAN as an end point, and the PDN-GW is a gateway having PDN as an end point.
The radio interface protocol layers between UE and a network can be divided into a first layer (L1), a second layer (L2) and a third layer (L3) based on three lower layers of an Open System Interconnection (OSI) reference model widely known in communications systems. A physical layer belonging to the first layer provides information transfer services using a physical channel, and a radio resource control (hereinafter, referred to as “RRC”) layer located at the third layer plays a role of controlling radio resources between UE and a network. For the purpose of this, the RRC layer exchanges RRC messages between UE and a network.
FIGS. 2 and 3 are views illustrating an architecture of a radio interface protocol between UE and a base station based on the 3GPP radio access network standard. The radio interface protocol horizontally includes a physical layer, a data link layer, and a network layer, and vertically divided into a user plane (U-plane) for transmitting data information and a control plane (C-plane) for transferring a control signaling. The protocol layers of FIGS. 2 and 3 can be divided into a first layer (L1), a second layer (L2) and a third layer (L3) based on three lower layers of an Open System Interconnection (OSI) reference model widely known in communications systems. Those radio protocol layers exist as a pair in the UE and the E-UTRAN to perform a data transmission for the radio section.
Hereinafter, each layer in a radio protocol control plane of FIG. 2 and a radio protocol user plane of FIG. 3 will be described.
The physical layer (PHY) which is a first layer provides information transfer services to the upper layers using a physical channel. The PHY layer is connected to the upper Medium Access Control (MAC) layer through a transport channel, and data between the MAC layer and the PHY layer is transferred through the transport channel. At this time, the transport channel is roughly divided into a dedicated transport channel and a common transport channel based on whether or not the channel is shared. Furthermore, data is transferred between different PHY layers, i.e., between PHY layers at the transmitter and receiver sides.
Various layers exist in the second layer. First, the Medium Access Control (MAC) layer serves to map various logical channels to various transport channels, and also performs a logical channel multiplexing for mapping several logical channels to one transport channel. The MAC layer is connected to an upper Radio Link Control (RLC) layer through a logical channel, and the logical channel is roughly divided into a control channel for transmitting control plane information and a traffic channel for transmitting user plane information according to the type of information to be transmitted.
The Radio Link Control (RLC) layer of the second layer manages segmentation and concatenation of data received from an upper layer to appropriately adjusts a data size such that a lower layer can send data to a radio section. Also, the RLC layer provides three operation modes such as a transparent mode (TM), an un-acknowledged mode (UM) and an acknowledged mode (AM) so as to guarantee various quality of services (QoS) required by each radio bearer (RB). In particular, AM RLC performs a retransmission function through an automatic repeat and request (ARQ) function for reliable data transmission.
A Packet Data Convergence Protocol (PDCP) layer of the second layer performs a header compression function for reducing the size of an IP packet header which is relatively large in size and contains unnecessary control information to efficiently transmit IP packets, such as IPv4 or IPv6, over a radio section with a relatively small bandwidth. Due to this, information only required from the header portion of data is transmitted, thereby serving to increase the transmission efficiency of the radio section. In addition, in the LTE system, the PDCP layer performs a security function, which includes ciphering for preventing the third person's data wiretapping and integrity protection for preventing the third person's data manipulation.
A radio resource control (RRC) layer located at the uppermost portion of the third layer is only defined in the control plane. The RRC layer performs a role of controlling logical channels, transport channels and physical channels in relation to configuration, re-configuration, and release of Radio Bearers (RBs). Here, the RB denotes a logical path provided by the first and the second layers for transferring data between the UE and the UTRAN. In general, the establishment of the RB refers to a process of stipulating the characteristics of protocol layers and channels required for providing a specific service, and setting each of the detailed parameter and operation method thereof. The RB is divided into a signaling RB (SRB) and a data RB (DRB), wherein the SRB is used as a path for transmitting RRC messages in the C-plane while the DRB is used as a path for transmitting user data in the U-plane.
Hereinafter, an RRC mode and an RRC connection method of UE will be described in detail. The RRC mode refers to whether or not the RRC of the UE is logically connected to the RRC of an E-UTRAN. If connected, then it is called an RRC_CONNECTED mode, and otherwise it is called an RRC_IDLE mode. For the UE in an RRC_CONNECTED mode, the E-UTRAN can recognize the existence of the relevant UE in a cell unit because there exist an RRC connection thereof, and thus the E-UTRAN can effectively control the UE. On the contrary, for the UE in RRC_IDLE mode, the E-UTRAN cannot recognize the relevant UE, and therefore, it is managed by a core network in a tracking area unit, which is a unit larger than a cell. In other words, the existence of the UE in an RRC_IDLE mode is only recognized in a large area unit, and therefore, it should be changed to an RRC_CONNECTED mode in order to receive typical mobile communication services such as voice or data.
When the UE is initially turned on by a user, the UE first searches for a suitable cell and then is camped in an RRC_IDLE mode in the corresponding cell. The UE camped in an RRC_IDLE mode makes an RRC connection with the RRC of the E-UTRAN through an RRC connection procedure when it is required to make an RRC connection, thereby changing the state to an RRC_CONNECTED mode. There are several cases when the UE in an idle mode is required to make an RRC connection. For example, an uplink data transmission may be required due to a phone call attempt by the user, or the like, or the transmission of a response message may be required in response to a paging message received from the E-UTRAN.
Hereinafter, Minimization Driving Test (MDT) will be described. A principal purpose of MDT is to change a test, which was executed using an actual vehicle by the existing operators, into an efficient scheme, in order to ensure cell coverage optimization. That is, one scheme is to detect a coverage hole. Coverage depends on a construction of a new base station or building or a user's usage environment. Thus, an operator must periodically execute a driving test, which causes consumption of a lot of costs and resources. MDT has a concept of using users' terminals instead of actual coverage measurement by the operators.